10,032 research outputs found
Nongaussian fluctuations arising from finite populations: Exact results for the evolutionary Moran process
The appropriate description of fluctuations within the framework of
evolutionary game theory is a fundamental unsolved problem in the case of
finite populations. The Moran process recently introduced into this context
[Nowak et al., Nature (London) 428, 646 (2004)] defines a promising standard
model of evolutionary game theory in finite populations for which analytical
results are accessible. In this paper, we derive the stationary distribution of
the Moran process population dynamics for arbitrary games for the
finite size case. We show that a nonvanishing background fitness can be
transformed to the vanishing case by rescaling the payoff matrix. In contrast
to the common approach to mimic finite-size fluctuations by Gaussian
distributed noise, the finite size fluctuations can deviate significantly from
a Gaussian distribution.Comment: 4 pages (2 figs). Published in Physical Review E (Rapid
Communications
Three-Omega Thermal-Conductivity Measurements with Curved Heater Geometries
The three-omega method, a powerful technique to measure the thermal
conductivity of nanometer-thick films and the interfaces between them, has
historically employed straight conductive wires to act as both heaters and
thermometers. When investigating stochastically prepared samples such as
two-dimensional materials and nanomembranes, residue and excess material can
make it difficult to fit the required millimeter-long straight wire on the
sample surface. There are currently no available criteria for how diverting
three-omega heater wires around obstacles affects the validity of the thermal
measurement. In this Letter, we quantify the effect of wire curvature by
performing three-omega experiments with a wide range of frequencies using both
curved and straight heater geometries on SiO/Si samples. When the heating
wire is curved, we find that the measured Si substrate thermal conductivity
changes by only 0.2%. Similarly, we find that wire curvature has no significant
effect on the determination of the thermal resistance of a 65 nm SiO
layer, even for the sharpest corners considered here, for which the largest
measured ratio of the thermal penetration depth of the applied thermal wave to
radius of curvature of the heating wire is 4.3. This result provides useful
design criteria for three-omega experiments by setting a lower bound for the
maximum ratio of thermal penetration depth to wire radius of curvature.Comment: 4 pages, 3 figure
Darwinian Selection and Non-existence of Nash Equilibria
We study selection acting on phenotype in a collection of agents playing
local games lacking Nash equilibria. After each cycle one of the agents losing
most games is replaced by a new agent with new random strategy and game
partner. The network generated can be considered critical in the sense that the
lifetimes of the agents is power law distributed. The longest surviving agents
are those with the lowest absolute score per time step. The emergent ecology is
characterized by a broad range of behaviors. Nevertheless, the agents tend to
be similar to their opponents in terms of performance.Comment: 4 pages, 5 figure
Spin Readout and Initialization in a Semiconductor Quantum Dot
Electron spin qubits in semiconductors are attractive from the viewpoint of
long coherence times. However, single spin measurement is challenging. Several
promising schemes incorporate ancillary tunnel couplings that may provide
unwanted channels for decoherence. Here, we propose a novel spin-charge
transduction scheme, converting spin information to orbital information within
a single quantum dot by microwave excitation. The same quantum dot can be used
for rapid initialization, gating, and readout. We present detailed modeling of
such a device in silicon to confirm its feasibility.Comment: Published versio
Temperature driven to phase-transformation in Ti, Zr and Hf from first principles theory combined with lattice dynamics
Lattice dynamical methods used to predict phase transformations in crystals
typically deal with harmonic phonon spectra and are therefore not applicable in
important situations where one of the competing crystal structures is unstable
in the harmonic approximation, such as the bcc structure involved in the hcp to
bcc martensitic phase transformation in Ti, Zr and Hf. Here we present an
expression for the free energy that does not suffer from such shortcomings, and
we show by self consistent {\it ab initio} lattice dynamical calculations
(SCAILD), that the critical temperature for the hcp to bcc phase transformation
in Ti, Zr and Hf, can be effectively calculated from the free energy difference
between the two phases. This opens up the possibility to study quantitatively,
from first principles theory, temperature induced phase transitions.Comment: 4 pages, 3 figure
Backflow in a Fermi Liquid
We calculate the backflow current around a fixed impurity in a Fermi liquid.
The leading contribution at long distances is radial and proportional to 1/r^2.
It is caused by the current induced density modulation first discussed by
Landauer. The familiar 1/r^3 dipolar backflow obtained in linear response by
Pines and Nozieres is only the next to leading term, whose strength is
calculated here to all orders in the scattering. In the charged case the
condition of perfect screening gives rise to a novel sum rule for the phase
shifts. Similar to the behavior in a classical viscous liquid, the friction
force is due only to the leading contribution in the backflow while the dipolar
term does not contribute.Comment: 4 pages, 1 postscript figure, uses ReVTeX and epsfig macro, submitted
to Physical Review Letter
Pressure Induced Charge Disproportionation in LaMnO
We present a total energy study as a function of volume in the cubic phase of
LaMnO. A charge disproportionated state into planes of
MnO/MnO was found. It is argued that the pressure
driven localisation/delocalisation transition might go smoothly through a
region of Mn and Mn coexistence.Comment: 3 pages, 1 figure, Conference Proceedings: Nanospintronics: Design
and Realization (Kyoto, Japan 24-28 May, 2004
MLS and CALIOP Cloud Ice Measurements in the Upper Troposphere: A Constraint from Microwave on Cloud Microphysics
This study examines the consistency and microphysics assumptions among satellite ice water content (IWC) retrievals in the upper troposphere with collocated A-Train radiances from Microwave Limb Sounder (MLS) and lidar backscatters from Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). For the cases in which IWC values are small (<10 mg m(-3)), the cloud ice retrievals are constrained by both MLS 240- and 640-GHz radiances and CALIOP 532-nm backscatter (532). From the observed relationships between MLS cloud-induced radiance T-cir and the CALIOP backscatter integrated (532) along the MLS line of sight, an empirical linear relation between cloud ice and the lidar backscatter is found: IWC/(532) = 0.58 +/- 0.11. This lidar cloud ice relation is required to satisfy the cloud ice emission signals simultaneously observed at microwave frequencies, in which ice permittivity is relatively well known. This empirical relationship also produces IWC values that agree well with the CALIOP, version 3.0, retrieval at values <10 mg m(-3). Because the microphysics assumption is critical in satellite cloud ice retrievals, the agreement found in the IWC-(532) relationships increase fidelity of the assumptions used by the lidar and microwave techniques for upper-tropospheric clouds
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